Integrated DC link choke and method for suppressing common-mode voltage in a motor drive

ABSTRACT

A motor drive and a choke therefor are disclosed, wherein the choke comprises a magnetic core with an inner leg and two outer legs, and four coils, which may be connected in DC current paths in the motor drive. First and second differential coils are wound around first and second outer legs, respectively; and first and second common-mode coils are wound around the inner leg of the choke. The first and second differential coils smooth the DC current on a DC bus in the motor drive, and the first and second common-mode coils suppress common-mode voltages in the motor drive. Also disclosed is a method for suppressing common-mode voltages in a motor drive.

TECHNICAL FIELD

The invention relates generally to the art of electric motors and motordrives and more particularly to an integrated DC link choke andmethodology for suppressing common-mode voltage and smoothing DC currentin a motor drive.

BACKGROUND OF THE INVENTION

In the field of electric motors and motor drives, a motor is connectedto a motor drive, which provides electrical power to the motor in acontrolled fashion, wherein the motor drive may take a variety of forms,such as a current source inverter (CSI), voltage source inverter (VSI)or the like. Such motor drives are commonly employed to provideelectrical power to motors. The motor drive may be employed in order toprovide speed control, torque control, and/or to control other motorperformance characteristics. For AC motors, electrical power isconverted in the motor drive from supply power, typically AC power froma utility or other source, into DC. The DC power is then converted, forexample using an inverter stage, into AC power at a controlled frequencyand amplitude, which is provided to the motor windings.

The AC to DC converter stage in the motor drive, and/or the subsequentinverter stage, often comprise solid state semiconductor-based switchingelements, such as silicon-controlled rectifiers (SCRs), gate turn-offthyristors (GTOs), gate commutated thyristors (GCTs), insulated gatebipolar transistors (IGBTs), or other switching devices. As theswitching elements are activated in the AC to DC converter stage or theinverter stage, common-mode voltages are produced. This common-modevoltage appears in the output phases of the motor drive, and hence, onthe motor windings. Where a neutral associated with the input powersource is grounded, this common-mode voltage appears between the motorwindings and ground, and may reach levels beyond the motor windinginsulation ratings. Consequently, such common-mode voltages may damagethe motor or decrease the life expectancy thereof.

Conventional approaches to addressing this motor drive common-modevoltage problem have included designing the motor winding insulation tohigher voltage ratings. However, this approach provides no solution forexisting motors, which do not include insulation capable of withstandingthe common-mode voltage levels. Another approach has been to provide anisolation transformer between the power source and the motor drive ACpower input terminals. However, such an isolation transformer must berated to handle the common-mode voltage levels. In addition, isolationtransformers add cost and occupy space in the motor drive system. Thus,there is a need for apparatus and techniques to reduce or minimize thecommon-mode voltages in a motor drive, which do not require isolationtransformers, and which operate in association with existing motors.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intendedneither to identify key or critical elements of the invention nor todelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

The invention provides a motor drive and a choke therefor, wherein thechoke comprises a magnetic core with an inner leg and two outer legs,and four coils, which may be connected in DC current paths in the motordrive. First and second differential coils are wound around first andsecond outer legs, respectively; and first and second common-mode coilsare wound around the inner leg of the choke. The first and seconddifferential coils smooth the DC power on a DC bus in the motor drive,and the first and second common-mode coils suppress common-mode voltagesin the motor drive. The invention thus provides an integrated DC linkchoke, which may be employed to provide both DC current smoothing andcommon-mode voltage suppression in a motor drive. In addition, theinvention provides for a solution to the above motor drive common-modevoltage problems, whereby motors need not include extra insulationbeyond the normal motor ratings, and whereby no external isolationtransformers are required for a motor drive system.

According to one aspect of the invention, a choke provides adifferential inductance and suppresses common-mode voltages in a motordrive having an AC to DC converter stage for converting AC power to DCpower on a DC bus with positive and negative DC current paths, and aninverter stage for converting DC power from the DC bus to AC motor powerin a controlled fashion. The choke comprises a magnetic core having aninner leg extending between an upper member and a lower member, andfirst and second outer legs extending from the lower member toward theupper member. A first air gap is provided between the first outer legand the upper member and a second air gap is provided between the secondouter leg and the upper member.

First and second differential coils are wound around the first andsecond outer legs, respectively, and may be connected in the positiveand negative DC current paths of the motor drive, respectively, tosmooth the DC power on the DC bus. The differential coils may be woundon the core to provide a differential flux in a differential flux paththrough the first and second outer legs, the first and second air gaps,and the upper and lower members of the core in response to differentialcurrent in the positive and negative DC current paths of the motordrive, whereby a differential inductance is provided to smooth DC poweron the motor drive DC bus.

In addition, first and second common-mode coils are wound around theinner leg of the choke, which may also be connected in the positive andnegative DC current paths, respectively, to suppress common-modevoltages in the motor drive. The common-mode coils provide a firstcommon-mode flux in a common-mode flux path through the inner leg, thefirst and second outer legs, the first and second air gaps, and theupper and lower members of the core in response to common-mode currentin the positive and negative DC current paths. The common-mode coilsthus provide a common-mode inductance to suppress common-mode voltagesin the motor drive. In addition, the differential coils may be woundaround the outer legs of the core in order to provide a secondcommon-mode flux in the common-mode flux path in response to common-modecurrent in the positive and negative DC current paths. The common-modecoils and the differential coils may thus provide a common-modeinductance to suppress common-mode voltages in the motor drive. Thecommon-mode inductance and the differential inductance may be adjustedindependently of one another according to a particular motor drivedesign.

Another aspect of the invention provides a motor drive for providing ACpower to a motor in a controlled fashion. The motor drive comprises anAC to DC converter stage for converting AC power to DC power on a DC buswith positive and negative DC current paths, and an inverter stage forconverting DC power from the DC bus to AC motor power in a controlledfashion. The motor drive further comprises a choke for providing adifferential inductance and suppressing common-mode voltages in themotor drive. The choke comprises a magnetic core having an inner legextending between an upper member and a lower member, and first andsecond outer legs extending from the lower member toward the uppermember and providing a first air gap between the first outer leg and theupper member and a second air gap between the second outer leg and theupper member. First and second differential coils are wound around thefirst and second outer legs, respectively, and first and secondcommon-mode coils are wound around the inner leg. The first and seconddifferential coils are connected in the positive and negative DC currentpaths, respectively, to smooth the DC power on the DC bus, and the firstand second common-mode coils are connected in the positive and negativeDC current paths, respectively, to suppress common-mode voltages in themotor drive.

Yet another aspect of the invention provides a methodology by whichcommon-mode voltages may be suppressed in a motor drive. A magnetic coreis provided having an inner leg and two outer legs. First and seconddifferential coils are wound around the first and second outer legs,respectively, and first and second common-mode coils are wound aroundthe inner leg. The first differential and common-mode coils areconnected in a positive DC current path in the motor drive, and thesecond differential and common-mode coils are connected in a negative DCpath, so as to suppress common-mode voltages in the motor drive.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described. The followingdescription and the annexed drawings set forth in detail certainillustrative aspects of the invention. However, these aspects areindicative of but a few of the various ways in which the principles ofthe invention may be employed. Other aspects, advantages and novelfeatures of the invention will become apparent from the followingdetailed description of the invention when considered in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary motor drive forproviding AC power to a motor in a controlled fashion in accordance withan aspect of the present invention;

FIG. 2 is a side elevation view of an exemplary choke for providing adifferential inductance and suppressing common-mode voltages in a motordrive according to another aspect of the invention;

FIG. 3 is a side elevation view further illustrating the choke of FIG.2; and

FIG. 4 is a flow diagram illustrating an exemplary method of suppressingcommon-mode voltages in a motor drive according to another aspect of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The various aspects of the present invention will now be described withreference to the drawings, wherein like reference numerals are used torefer to like elements throughout. The invention provides a choke foruse in a motor drive, which comprises a magnetic core with an inner legand two outer legs, and four coils, which may be connected in DC currentpaths in the motor drive. First and second differential coils are woundaround first and second outer legs, respectively; and first and secondcommon-mode coils are wound around the inner leg of the choke. The firstand second differential coils smooth the DC power on a DC bus in themotor drive, and the first and second common-mode coils suppresscommon-mode voltages in the motor drive. In addition, the differentialcoils may also suppress common-mode voltages. The invention thusprovides an integrated DC link choke, which may be employed to provideboth DC current smoothing and common-mode voltage suppression in a motordrive.

Referring now to FIG. 1, an exemplary motor drive 2 is illustrated forproviding AC electric power to an electric motor 4 in a controlledfashion, wherein the motor 4 may be an induction or other type of ACmotor. Electric power is provided to the drive 2 from a three phaseinput power source 6 comprising AC power sources 7 a, 7 b, and 7 c,interconnected in a “Y” arrangement, with a grounded neutral. Threephase AC power from the input power source 6 is provided to an AC to DCconverter stage 8 comprising switching elements S1, S2, S3, S4, S5, andS6, which may be selectively actuated by control signals (not shown)thereto in order to generate DC power on a DC bus 10 having a positiveDC current path 12 and a negative DC current path 14 in a manner knownin the art. DC power from the bus 10 is then converted to AC motor powervia an inverter stage 20 comprising switching elements S7, S8, S9, S10,S11, and S12. The switching elements S7-S12 may be activated accordingto control signals (not shown) in a manner known in the art in order toprovide AC power of appropriate amplitude and phase to the motor 4 in acontrolled fashion.

The motor drive 2 further comprises a choke 30 for providing adifferential inductance to smooth DC power on the bus 10, and also tosuppress common-mode voltages in the motor drive 2 in accordance with anaspect of the invention. The choke 30 comprises first and seconddifferential coils A and B connected in the positive and negative DCcurrent paths 12 and 14, respectively, to smooth the DC power on the DCbus 10. In addition, choke 30 comprises first and second common-modecoils C and D connected in the positive and negative DC current paths 12and 14, respectively, to suppress common-mode voltages resulting fromswitching of one or more switching elements S1-S12 in the motor drive 2.Coils A and C are connected in series in the positive DC current path 12with terminal A1 of coil A connected to switches S1-S3 of the AC to DCconverter stage 8, terminal A2 of coil A connected to Terminal C2 ofcoil C, and terminal C1 of coil C connected to switches S7-S9 of theinverter stage 20, whereby DC current Id flows through the coils A and Cin the positive DC current path 12 of the DC bus 10.

Coils B and D are connected in series in the negative DC current path 14with terminal B1 of coil B connected to switches S4-S6 of the AC to DCconverter stage 8, terminal B2 of coil B connected to Terminal D2 ofcoil D, and terminal D1 of coil D connected to switches 510-S12 of theinverter stage 20. Thus connected, DC current Id flows through the coilsB and D in the negative DC current path 14 of the DC bus 10. By theinclusion of the choke 30, the motor drive 2 provides for smoothing ofripple current on the DC bus 10 via a differential inductance providedto the DC current Id by the differential coils A and B, as well assuppression of common-mode voltages via a common-mode inductanceprovided by the differential coils C and D. In this regard, it will benoted that common-mode current I_(CM) flows through the positive andnegative current paths 12 and 14 in the directions indicated in FIG. 1.In addition, the first and second differential coils A and B may providefurther suppression of common-mode voltages in the motor drive 2.

Other forms and topologies of motor drives are possible within the scopeof the present invention, including the exemplary motor drive 2 of FIG.1 as well as other motor drives not illustrated and described herein.For example, the motor drive 2 may be modified by inclusion of lineinductors between the single phase power sources 7 a, 7 b, and 7 c, andthe switching elements S1, S2, S3, S4, S5, and S6 of the AC to DCconverter stage 8, output capacitors connected between the motor phasewindings and a common neutral, and/or a capacitance connected across theDC bus 10.

Referring now to FIGS. 2 and 3, an exemplary choke 130 is illustrated,which may be employed to provide a differential inductance and suppresscommon-mode voltages in a motor drive in accordance with another aspectof the invention. For example, the choke 30 may be employed in a motordrive, such as drive 2 of FIG. 1, having an AC to DC converter stage forconverting AC power to DC power on a DC bus with positive and negativeDC current paths, and an inverter stage for converting DC power from theDC bus to AC motor power in a controlled fashion, although the choke 130finds application in association with motor drives and power convertersapart from those illustrated and described herein.

The choke 130 comprises a magnetic core 132 having an inner leg 134extending between an upper member 136 and a lower member 138. While theexemplary choke core 132 is illustrated as having a generallyrectangular profile, core profiles and orientations other than thoseillustrated and described herein are contemplated as falling within thescope of the invention, for example, rounded profiles, and the like. Thecore 132 may be made of any suitable material for creating a choke, asis known in the art. A first outer leg 140 extends from the lower member138 toward the upper member 136 with a first air gap 142 therebetween,and a second outer leg 144 extends from the lower member 138 toward theupper member 136 with a second air gap 146 between the second outer leg144 and the upper member 136.

The choke 130 further comprises a first differential coil A wound aroundthe first outer leg 140, and a second differential coil B wound aroundthe second outer leg 144. The differential coils A and B, as well as theexemplary common-mode coils C and D described hereinafter, may be madeof any suitable material as is known for conducting current, and mayinclude appropriate insulation. Furthermore, the number of turns of thevarious coils may be determined according to a particular application,such as a specific motor drive or power conversion system. Thedifferential coils A and B may be used for smoothing DC current in amotor drive as illustrated and described in greater detail hereinafter.For instance, the first and second differential coils A and B may beconnected in positive and negative DC current paths, respectively, in amotor drive (e.g., motor drive 2 of FIG. 1) in order to smooth DCcurrent in the drive. In addition, the choke 130 comprises first andsecond common-mode coils C and D, respectively, wound around the innerleg 134. The common-mode coils C and D may provide suppression orreduction in common-mode voltages in a motor drive in accordance withthe invention. For example, the first and second common-mode coils C andD may be connected in the positive and negative DC current paths,respectively, to suppress common-mode voltages in the motor drive.

The coils A, B, C, and D of the choke 130 may be connected in a motordrive in a fashion similar to that of choke 30 in the motor drive 2 ofFIG. 1 in order to reduce common-mode voltages and smooth DC currenttherein. Thus, a differential DC current Id may flow into a terminal A1of coil A, from terminal A2 of coil A into a terminal C2 of coil C, andout of terminal C1 of coil C. For instance, the terminals A2 and C2 maybe connected together, with terminals A1 and C1 connected in a positiveDC current path of a DC power bus in a motor drive (e.g., path 12 ofdrive 2). Similarly, a DC current Id may flow in a return or negativecurrent path (e.g., path 14 of drive 2) into a terminal D1 of coil D,from a terminal D2 of coil D into a terminal B2 of coil B, and out of aterminal B1 of coil B. Thus, the terminals D2 and B2 may be connectedtogether, with terminals D1 and B1 connected in a negative or return DCcurrent path of a DC power bus in a motor drive.

Thus connected, the choke 130 advantageously provides a differentialinductance to smooth DC current in a motor drive DC bus, as well as acommon-mode inductance to suppress common-mode voltages in the motordrive. Where the number of turns of coils A, B, C, and D are the same,the common-mode inductance is about 2.25 times the differentialinductance. In addition, the common-mode and differential inductancesmay be independently adjusted for a particular motor drive or otherpower conversion application, for example, through adjusting therelative number of turns in the coils A, B, C, and D. As illustrated inFIG. 2, the first and second differential coils A and B providedifferential fluxes Φ_(d−A) and Φ_(d−B) in a differential flux paththrough the first and second outer legs 140 and 144, the first andsecond air gaps 142 and 146, and the upper and lower members 136 and 138of the core 132 in response to a differential current Id in the positiveand negative DC current paths. It will be noted that the first andsecond common-mode coils C and D are wound around the inner leg 134 soas to provide substantially zero differential flux in the core 132 inresponse to differential current Id, wherein the differential fluxesΦ_(d−C) and Φ_(d−D) related thereto in the inner leg 134 cancel eachother.

Referring also to FIG. 3, the first and second common-mode coils C and Dfurther provide a first common-mode flux comprising (Φ_(CM−C) andΦ_(CM−D) in a common-mode flux path through the inner leg 134, the firstand second outer legs 140 and 144, the first and second air gaps 142 and146, and the upper and lower members 136 and 138, respectively, of thecore 132 in response to common-mode current I_(CM) in the positive andnegative DC current paths of the motor drive. In this case, it is notedthat whereas the differential current Id flows into terminal D1 and outof terminal B1 (e.g., FIGS. 1 and 2), the common-mode current I_(CM)/2flows into terminal B1 and out of terminal D1 (e.g., FIGS. 1 and 3). Inaddition, the first and second differential coils A and B provide asecond common-mode flux comprising Φ_(CM−A) and Φ_(CM−B) in thecommon-mode flux path in response to common-mode current in the positiveand negative DC current paths.

As illustrated in FIGS. 2 and 3, therefore, the common-mode coils C andD and the differential coils A and B provide a common-mode inductance tosuppress common-mode voltages in the motor drive, whereas thedifferential coils A and B provide a differential inductance to smooththe DC current on the DC bus. Due to the winding of the common-modecoils C and D around the inner leg 134, substantially no differentialinductance is provided by the coils C and D.

The exemplary integrated choke 130 of FIGS. 2 and 3 has been found toprovide significant advantages in suppressing common-mode voltages inmotor drives. In this regard, the ratio between the common-modeinductance provided by coils A, B, C, and D to the differentialinductance provided by coils A and B facilitates the use of the choke130 in suppressing common-mode voltages while smoothing DC power in themotor drive DC bus. The integration of the four windings A, B, C, and D,moreover, provides for induction ratios (e.g., common-mode todifferential) higher than those attainable using separate differentialand common-mode chokes with an equivalent number of turns.

For example, referring to FIG. 2 and assuming each of the coils A, B, C,and D comprises an integer number N turns, the differential inductanceLd in the exemplary choke 130 is given by the following equation (1):

Ld=(N _(A) +N _(B))² /R=2N ² /R;  (1)

where R is the reluctance of the core 132, and where N_(A)=N_(B)=N.

Referring to FIG. 3, and again assuming each coil A, B, C, and D has aninteger number N turns, the common-mode inductance L_(CM−AC) produced bythe coils A and C in the exemplary choke 130 is given by the followingequation (2):

L _(CM−AC) =L _(A) +L _(C)=3N ² /R+6N ² /R=9N ² /R;  (2)

and the common-mode inductance L_(CM−BD) produced by the coils B and Din the choke 130 is given by the following equation (3):

L _(CM−BD) =L _(B) +L _(D)=3N ² /R+6N ² /R=9N ² /R.  (3)

The common-mode inductances L_(CM−AC) and L_(CM−BD) are effectively inparallel. Thus, the total common-mode inductance L_(CM) produced by thecoils A, B, C, and D of the choke 130 is given by the following equation(4):

L _(CM) =L _(CM−AC) //L _(CM−BD)=9N ² /R//9N ² /R=9N ²/2R.  (4)

Accordingly, the ratio of the common-mode inductance L_(CM) for thechoke 130 to the differential inductance Ld is given by the followingequation (5):

L _(CM) /Ld=(9N ²/2R)/(2N ² /R)=9/4=2.25.  (5)

It will be appreciated that other common-mode to differential inductanceratios are obtainable within the scope of the invention, for example,via adjustment of the relative number of turns in the respective coilsA, B, C, and D.

Another aspect of the invention provides methodologies by whichcommon-mode voltages may be reduced or suppressed in a motor drive. Onesuch method 200 is illustrated in FIG. 4. While the exemplary method 200is illustrated and described herein as a series of blocks representativeof various events and/or acts, the present invention is not limited bythe illustrated ordering of such blocks. For instance, some acts orevents may occur in different orders and/or concurrently with other actsor events, apart from the ordering illustrated herein, in accordancewith the invention. Moreover, not all illustrated blocks, events, oracts, may be required to implement a methodology in accordance with thepresent invention. In addition, it will be appreciated that theexemplary method 200 and other methods according to the invention, maybe implemented in association with the apparatus and systems illustratedand described herein, as well as in association with other systems andapparatus not illustrated or described.

Beginning at 202, a magnetic core is provided, which comprises an innerleg and first and second outer legs. The inner leg may extend betweenupper and lower members, and the first and second outer legs may extendfrom the lower member toward the upper member, with first and second airgaps in between. At 204, first and second differential coils are woundaround the first and second outer legs, respectively, of the core, afterwhich first and second common-mode coils are wound around the inner legat 206. At 208, the first and second differential coils are connected inpositive and negative DC current paths in a motor drive, respectively,and at 210 the first and second common-mode coils are respectivelyconnected in the positive and negative DC current paths. The method 200thus provides for reducing or suppressing common-mode voltages in amotor drive by the configuration and connection of the differential andcommon mode coils with respect to the core and the motor drive.

Although the invention has been shown and described with respect tocertain implementations, it will be appreciated that equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components (assemblies, devices, circuits, systems,etc.), the terms (including a reference to a “means”) used to describesuch components are intended to correspond, unless otherwise indicated,to any component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure, which performs thefunction in the herein illustrated exemplary applications andimplementations of the invention.

What has been described above includes examples of the presentinvention. It is, of course, not possible to describe every conceivablecombination of components or methodologies for purposes of describingthe present invention, but one of ordinary skill in the art mayrecognize that many further combinations and permutations of the presentinvention are possible. Accordingly, the present invention is intendedto embrace all such alterations, modifications and variations that fallwithin the spirit and scope of the appended claims. In addition, while aparticular feature of the invention may have been disclosed with respectto only one of several aspects or implementations of the invention, sucha feature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the terms“includes”, “including”, “has”, “having”, and variants thereof are usedin either the detailed description or the claims, these terms areintended to be inclusive in a manner similar to the term “comprising”and its variants.

What is claimed is:
 1. A choke for providing a differential inductanceand suppressing common-mode voltages in a motor drive having an AC to DCconverter stage for converting AC power to DC power on a DC bus withpositive and negative DC current paths, and an inventor stage forconverting DC power from the DC bus to AC motor power in a controlledfashion, the choke comprising: a magnetic core having an inner legextending between an upper member and a lower member, and first andsecond outer legs extending from the lower member toward the uppermember and providing a first air gap between the first outer leg and theupper member and a second air gap between the second outer leg and theupper member; first and second differential coils wound around the firstand second outer legs, respectively; and first and second common-modecoils wound around the inner leg; wherein the first and seconddifferential coils are adapted for connection in the positive andnegative DC current paths, respectively, to smooth the DC power on theDC bus, and wherein the first and second common-mode coils arc adaptedfor connection in the positive and negative DC current paths,respectively, to suppress common-mode voltages in the motor drive. 2.The choke of claim 1, wherein the first and second common-mode coils arewound around the inner leg to provide substantially zero differentialflux in the core in response to differential current in the positive andnegative DC current paths.
 3. The choke of claim 2, wherein the firstand second differential coils provide a differential flux in adifferential flux path through the first and second outer legs, thefirst and second air gaps, and the upper and lower members of the corein response to differential current in the positive and negative DCcurrent paths.
 4. The choke of claim 3, wherein the first and secondcommon-mode coils provide a first common-mode flux in a common-mode fluxpath through the inner leg, the first and second outer legs, the firstand second air gaps, and the upper and lower members of the core inresponse to common-mode current in the positive and negative DC currentpaths.
 5. The choke of claim 4, wherein the first and seconddifferential coils provide a second common-mode flux in the common-modeflux path in response to common-mode current in the positive andnegative DC current paths.
 6. The choke of claim 5, wherein the firstand second common-mode coils and the first and second differential coilsprovide a common-mode inductance to suppress common-mode voltages in themotor drive.
 7. The choke of claim 6, wherein the first and seconddifferential coils provide a differential inductance to smooth the DCcurrent on the DC bus.
 8. The choke of claim 7, wherein common-modeinductance is about 2.25 times the differential inductance.
 9. The chokeof claim 7, wherein the common-mode inductance and the differentialinductance are independently adjustable.
 10. The choke of claim 1,wherein the first and second differential coils provide a differentialflux in a differential flux path through the first and second outerlegs, the first and second air gaps, and the upper and lower members ofthe core in response to differential current in the positive andnegative DC current paths.
 11. The choke of claim 10, wherein the firstand second common-mode coils provide a first common-mode flux in acommon-mode flux path through the inner leg, the first and second outerlegs, the first and second air gaps, and the upper and lower members ofthe core in response to common-mode current in the positive and negativeDC current paths.
 12. The choke of claim 1, wherein the first and secondcommon-mode coils provide a first common-mode flux in a common-mode fluxpath through the inner leg, the first and second outer legs, the firstand second air gaps, and the upper and lower members of the core inresponse to common-mode current in the positive and negative DC currentpaths.
 13. The choke of claim 12, wherein the first and seconddifferential coils provide a second common-mode flux in the common-modeflux path in response to common-mode current in the positive andnegative DC current paths.
 14. The choke of claim 1, wherein the firstand second common-mode coils and the first and second differential coilsprovide a common-mode inductance to suppress common-mode voltages in themotor drive.
 15. The choke of claim 14, wherein the first and seconddifferential coils provide a differential inductance to smooth the DCpower on the DC bus.
 16. The choke or claim 15, wherein common-modeinductance is about 2.25 times the differential inductance.
 17. Thechoke of claim 15, wherein the commnon-mode inductance and thedifferential inductance are independently adjustable.
 18. The choke ofclaim 1, further comprising a motor drive having at least one of: an A/Dconverter; a first and a second winding for smoothing DC power; a thirdand fourth winding for suppressing common-mode voltages, and a D/Aconverter.
 19. A motor drive for providing AC power to a motor in acontrolled fashion, comprising: an AC to DC converter stage forconverting AC power to DC power on a DC bus with positive and negativeDC current paths; an inverter stage for converting DC power from the DCbus to AC motor power in a controlled fashion; and a choke for providinga differential inductance and suppressing common-mode voltages in themotor drive, the choke comprising: a magnetic core having an inner legextending between an upper member and a lower member, and first andsecond outer legs extending from the lower member toward the uppermember and providing a first air gap between the first outer leg and theupper member and a second air gap between the second outer leg and theupper member, first and second differential coils wound around the firstand second outer legs, respectively; and first and second commnon-modecoils wound around the inner leg; wherein the first and seconddifferential coils are connected in the positive and negative DC currentpaths, respectively, to smooth the DC current on the DC bus, and whereinthe first and second common-mode coils are connected in the positive andnegative DC current paths, respectively, to suppress common-modevoltages in the motor drive.
 20. The motor drive of claim 19, whereinthe first and second common-mode coils are wound around the inner leg toprovide substantially zero differential flux in the core in response todifferential current in the positive and negative DC current paths. 21.The motor drive of claim 19, wherein the first and second differentialcoils provide a differential flux in a differential flux path throughthe first and second outer legs, the first and second air gaps, and theupper and lower members of the core in response to differential currentin the positive and negative DC current paths.
 22. The motor drive ofclaim 21, wherein the first and second common-mode coils provide a firstcommon-mode flux in a common-mode flux path through the inner leg, thefirst and second outer legs, the first and second air gaps, and theupper and lower members of the core in response to common-mode currentin the positive and negative DC current paths.
 23. The motor drive ofclaim 19, wherein the first and second common-mode coils provide a firstcommon-mode flux in a common-mode flux path through the inner leg, thefirst and second outer legs, the first and second air gaps, and theupper and lower members of the core in response to common-mode currentin the positive and negative DC current paths.
 24. The motor drive ofclaim 23, wherein the first and second differential coils provide asecond common-mode flux in the common-mode flux path in response tocommon-mode current in the positive and negative DC current paths. 25.The motor drive of claim 19, wherein the first and second common-modecoils and the first and second differential coils provide a common-modeinductance to suppress common-mode voltages in the motor drive.
 26. Themotor drive of claim 25, wherein the first and second differential coilsprovide a differential inductance to smooth the DC power on the DC bus.27. The motor drive of claim 26, wherein common-mode inductance is about2.25 times the differential inductance.
 28. The motor drive of claim 26,wherein the common-mode inductance and the differential inductance areindependently adjustable.
 29. A motor drive for providing AC power to amotor in a controlled fashion, comprising: an AC to DC converter stagefor converting AC power to DC power on a DC bus with positive andnegative DC current paths; an inverter stage for converting DC powerfrom the DC bus to AC motor power in a controlled fashion; and a chokehaving first and second differential coils connected in the positive andnegative DC current paths, respectively, to smooth the DC current on theDC bus, and first and second common-mode coils connected in the positiveand negative DC current paths, respectively, to suppress common-modevoltage in the motor drive.
 30. A method for suppressing common-modevoltages in a motor drive, comprising: providing a magnetic core havingan inner leg extending between an upper member and a lower member, andfirst and second outer legs extending from the lower member toward theupper member, and with a first air gap between the first outer leg andthe upper member and a second air gap between the second outer leg andthe upper member; winding first and second differential coils around thefirst and second outer legs of the magnetic core, respectively; windingfirst and second common-mode coils around the inner leg of the magneticcore; connecting the first and second differential coils in positive andnegative DC current paths in the motor drive, respectively, so as tosmooth DC current in the motor drive; and connecting the first andsecond common-mode coils in the positive and negative DC current paths,respectively, so as to suppress common-mode voltages in the motor drive.